DESCRIPTION: The long term goal of the research proposed here is to understand the mechanisms by which social interactions influence the nervous system. Specifically, how do social encounters between individuals produce cell specific changes in the brain? This will be studied by exploiting the well defined social system of a teleost fish in which it is possible to manipulate key features of their social status and reproductive state under controlled laboratory conditions. The principal investigator proposes continuing his study of the forebrain magnocellular gonadotropin-releasing hormone neurons using cellular and molecular probes developed during the previous period of support. In this system, the principal investigator has shown that when males win a territorial encounter, cells which contain GnRH enlarge significantly in volume (ca.8x), and when territorial males lose such an encounter, these same cells shrink. This system is highly advantageous for analysis of the cellular consequences of social behavior. Forebrain immunoreactive (ir)-GnRH comprise a phylogenetically ancient system which plays a central role in modulating pituitary control of reproduction through gonadal maturation in all vertebrates. Although analysis of this system is complicated in mammals because of the superimposition of neocortical functions, basal forebrain organization can be readily studied in teleosts.